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A gamma-ray burst (GRB) blasting from an expired star in the distant Universe has lit up a pair of galaxies seen when the Universe was just 1.8 billion years old, revealing surprising details about their chemical makeup.

GRB 090323 was first detected on 23 March 2009 by NASA's Fermi space telescope and then the Swift satellite, shortly followed by the ground-based GROND system (Gamma-Ray burst Optical and Near-infrared Detector) at the MPG/ESO 2.2-metre telescope in Chile, as well as ESO's Very Large Telescope (VLT). The VLT observations revealed that the gamma-ray burst injected light through its host galaxy and another nearby galaxy, which are both seen at a redshift of 3.57, equivalent to 12 billion years ago.

Artist's impression of a gamma-ray burst (left) shooting light through its host galaxy and a nearby galaxy that could be interacting with the host. Observations show the two galaxies to be enriched in heavy chemical elements. Image: ESO/L. Calçada.

By analysing the way the GRB's light was absorbed by gas in the galaxies, the chemical ingredients of the galaxies' cool gas could be determined. "We detect the usual elements that are seen in distant galaxies, namely: zinc, iron, sulfur, silicon, but more abundant than normally," says Sandra Savaglio of the Max-Planck Institute for Extraterrestrial Physics in Germany. "The vast majority of the Sun is made of hydrogen and helium, all the other elements – we call them metals – are about 1.4% of the total (in number of particles). In our galaxies it looks that all the other elements, with respect to hydrogen and helium, are almost twice more abundant than in the Sun."

The finding is surprising since the early Universe comprised mostly light elements with elements heavier than hydrogen and helium synthesized by reactions inside giant stars and fed into the local interstellar medium when they exploded in blinding supernova explosions at the end of their lives. Subsequent generations of stars therefore become progressively more enriched in heavy elements, but the new results show that some galaxies are already very rich in these elements, even less than two billion years after the Big Bang. "The Universe looks more chemically evolved than generally thought," says Savaglio.

The galaxy pair must be churning out new stars at a rapid rate in order to enrich the gas so effectively. One way this can be achieved is if the galaxies are in the process of merging – the interaction of their gas clouds triggering pockets of high density material that collapse into new stars. The finding also suggests that merging galaxies in the distant Universe might trigger GRBs.

Savaglio adds that the observing power of future generation telescopes like the European Extremely Large Telescope (E-ELT) would provide deeper, more precise information about the galaxies and the primordial Universe in general. "At the moment, spectra of galaxies and GRBs are reasonable and certainly better than what was possible with four metre telescopes in the past; we couldn't do these observations 15 years ago," she says. "But distant targets are faint, and good spectra, as in our case, are rare. Our GRB was particularly bright. With E-ELT and an event as bright as GRB 090323, I can't imagine how much easier it would be to do the job we did with the VLT, plus we would extent our observations more in the near UV and in the near infrared."

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